Re: why does human body tremble under different circumstances?

Thanks for your excellent question.

The first part is easy to answer. The tremor that occurs when it is cold 
outside is usually called shivering. The muscles in your body alternately 
contract and relax. This activity causes the little movements that you are 
talking about when you are cold. This generates heat and helps keep the 
body warm.

The second type of tremor occurs when one is scared, under pressure, etc. 
When this happens, something called the "fight or flight" response is 
activiated. This is done by a part of your nervous system called the 
sympathetic nervous system. The job of the sympathetic nervous system is 
to help your body get out of trouble. So when you are threatened or scared 
or you are under pressure (big test, playing a soccer or football game), 
etc., your body reacts by increasing its heart rate and the amount of 
blood pumped, breathing fast, increasing blood flow to the muscles, and 
other things that prepare your body to either fight whatever is putting 
you in danger or get out of danger by running (the flight in fight or 
flight). One other effect of this is that your nervous system is also at a 
higher state of activation, and this causes your muscles to shake a little 
(the tremor).

So basically, when you are in danger, your body prepares for the danger. 
One of these things is to increase the activity of your nervous system. 
And part of this increase of activity is the tremor. 

Reference: This about the autonomic nervous system (the sympathetic 
nervous system is part of the automatic nervous system):  http://faculty.washington.edu/chudler/auto.html